Fatal car crashes that involved marijuana tripled in the past decade, a new study concludes. One in nine drivers involved in a fatal crash tests positive for marijuana, according to the Columbia University researchers.

“If this trend continues, in five or six years non-alcohol drugs will overtake alcohol to become the most common substance involved in deaths related to impaired driving,” said study co-author Dr. Guohua Li.

The researchers analysed crash statistics from six states that routinely perform toxicology tests on drivers involved in fatal crashes, HealthDay reports. The study included data on more than 23,500 drivers who died within an hour of a crash between 1999 and 2010. Throughout the decade, alcohol contributed to about 40 percent of crashes. Drugged driving accounted for about 16 percent of fatal crashes in 1999, and more than 28 percent in 2010.

Marijuana use contributed to about 4 percent of fatal crashes in 1999, and 12 percent in 2010. The combination of marijuana and alcohol is particularly dangerous, the researchers found. “If a driver is under the influence of alcohol, their risk of a fatal crash is 13 times higher than the risk of the driver who is not under the influence of alcohol,” Li said. “But if the driver is under the influence of both alcohol and marijuana, their risk increases to 24 times that of a sober person.” The findings appear in the American Journal of Epidemiology.

“Given the increasing availability of marijuana and the ongoing opioid overdose epidemic, understanding the role of controlled substances in motor vehicle crashes is of significant public health importance,” Li said in a news release.